The present invention relates to a tire building drum of the type having a pair of radially expansible rings for gripping the beads of a tire carcass, and more particularly to such rings which are radially expansible without the use of inflatable bladders. The rings may be used in either a single stage building drum, wherein the tire is both built and expanded into a toroidal shape on one drum, in a first stage drum for building but not expanding a tire carcass, or in a second stage shaping drum wherein a prebuilt tire carcass is expanded to the desired toroidal shape for further processing.
As is known to those skilled in the art, radial ply pneumatic tires are formed in either one or two stage processes. In the one stage process, the tire carcass is both built and expanded to a toroidal shape for breaker and tread application on a single drum, whereas in the two stage process, the tire carcass is formed on one building drum and then expanded to a toroidal shape on a second, shaping drum. In the one stage drum and in the drums of the two stage process for building radial ply tires, and in the drums for building bias ply tires, there is a requirement for a pair of rings to grip and hold the tire beads in position while the tire carcass is being built or while it is being expanded to the desired toroidal shape. The construction and design of these bead grip rings are such that tire beads of a tire carcass can be passed over them onto or off the drum when the bead grip rings are in the unexpanded, retracted position.
The concept of an extensible gripping means to support a tire bead is not novel. It is well known to use a series of metallic fingers, or sections, which expand and contract radially. One such method of expansion of the bead grip means is described in U.S. Pat. No. 3,489,634, issued to T. D. Pizzo et al. on Jan. 13, 1970 and assigned to the assignee of the present invention. Although this type of radially extensible bead gripping means is suitable for manufacture of bias ply tires, it does present aspects which could be improved for the manufacture of tires in general, for the reasons that the extensible, metallic segments or fingers are expensive, complex and require precisely machined components. Further, since the metallic segments or fingers comprise the periphery of the bead grip means, the outer radial edges must conform to a perfect circle when used to produce a tire of a specific bead diameter. If a tire of a different bead diameter were to be built, either a compromise in the radial segments would be necessary or the multi-segmented bead grip ring would have to be changed. Unless perfect matching between tire bead and grippers were obtained, local stresses would develop in the cord plies of the carcass. Still further, after extension of the bead grip, the segments or fingers separate and cause gaps in the circumference of the gripping rings. Since the carcass cords are displaced in a radial direction, the intermittent grip of the segment and the gap between them will cause uneven tension in the cords of the carcass along the periphery of the bead during the carcass shaping process. Also, the spaces between segments would be difficult to seal, were it desirable to retain air within the carcass when positioned on the bead grips.
Rubber-like extensible bead grip rings are also well known. They obviate the problems of the metallic segments or fingers, but they create new operational problems. The force for extending the rubber rings is generally provided by a rubber-like bladder or tube located in an annular space immediately under the extensible rubber rings. Since these rubber-like bladders are placed in a confined and restricted area, repeated expansion and contraction of the bladder generates frictional forces along the walls of the annular space which abrade and distort the bladder, thereby causing premature failure of the bladder.